2008 BMW 750LI body & structure problems

A SUV traveling at 20 miles per hr hits passenger side of a 750 li traveling crossing a 2 way intersection spins the cars in a circle with skid marks on the highway damage the passenger side of the door and the lower panel of car and only there bumper was loose and it was driveable how is that a SUV traveling at such slow speed as I been told can do such do as much damages to a 750 li and the car is totaled

After dropping my daughter (who was sitting in the rear seat of my car) to her school in the morning, I was driving to work. I confirmed that all the doors were closed and latched after she exited the car. Just before reaching my work location, the driver side rear door unexpectedly opened while the vehicle was in motion. After stopping, the open door could not be closed or latched again.